Steel sheet is used in a variety of applications which if unprotected leads to rapid corrosion and resultant loss in structural strength. The most common technique to protect steel sheet from corrosion is to coat the steel sheet with a metallic protective coating such as galvanized, galvalume or aluminized coating. The protective material is commonly applied to steel sheet by a hot dip process, painting or electroplating. Electroplating of zinc protective material to a steel sheet is useful when precise amounts of galvanize coating material is to be applied to complex surface configurations. However, such process is energy intensive and hence, expensive compared to the hot dip application process which is satisfactory for use in coating most types of flat sheet. In some instances the flat sheet is formed into the product for use such as animal drinking troughs, small culvert, auger chutes and the like. They are dipped directly in a hot tank of, galvanized material and removed to achieve a coating of galvanized material on the article. However, the thickness of the galvanized material can vary considerably across the surface of the article.
Exemplary of normal coating methods, sheet galvanizing is usually carried out in a hot dip bath where the sheet passes downwardly into the bath and then is drawn upwardly out of the bath in a vertical manner. The excess galvanized material which adheres to the sheet is removed by the use of high velocity air or nitrogen jets to provide the desired coating thickness on the galvanized sheet. In this way, all surfaces of the sheet are covered galvanized material. Usually special edge treatment is required because of the inherent processing problem that excess protective material, such as galvanize, builds up on the edges of the sheet. Such edge build up greatly detracts from its use in forming-various articles and hence requires removal before forming of the sheet into the article or as already noted, requires special edge treatment while the sheet is being removed from the hot dip tank.
It is normal to roughen the surface of the steel sheet in order to increase the quantity of protective metallic material adhered to the steel surface. As is appreciated, the zinc in the galvanized or the aluminum in the galvalume or aluminized material reacts with the iron in the steel sheet to form zinc-iron alloy and/or an aluminum iron alloy at the interface between the protective material and the steel sheet. Such alloy then adheres or bonds the solidified coating material to the steel sheet.
The usual technique for roughening the steel sheet is to use a surface finishing roller which has a rough surface provided thereon. The roller provides for the last pass once the sheet has been rolled to the desired thickness. During the last pass the roughened pattern on the roller is transferred to the steel sheet. The most common technique for roughening the surface of the roller in order to transfer a roughened pattern to the steel sheet is to expose the steel roller to shot blast. Various size grits of particles are used to obtain the desired surface roughness on the rollers. Such surface roughness is in the form of jagged peaks which transfer the corresponding image to the surface of the steel sheet to provide a roughened surface with several peaks and valleys. Due to the sharpness of the ridges on the rollers they tend to wear quickly resulting in an inferior roughened surface on the steel sheet and also requiring rejuvenation of the steel rollers by shot blasting.
This type of roughened surface has resulted in inconsistencies in coating weight of protective metallic material applied to the sheet surface. Such variation in the coating weight requires that the coated sheet material be downgraded for purposes of sale thereby resulting in significant loss of revenue. The downgraded reject material becomes more and more of problem as coating plants attempt to apply greater quantities of coating material to the surface of the sheets. For example, with existing processes, total sheet coating weights in the range of 600 to 700 g/m.sup.2, can result in up to 20 to 25% reject material due to inconsistency in coating thicknesses.
There continues however to be a significant demand for heavy coated steel sheet to provide greater corrosion resistance, and longer life. However, with existing processes such material is expensive to produce.
Most attempts at providing maximum surface roughness in the rolled steel sheet has resulted in failure in attempting to exceed galvanized or other metal coating quantities of approximately 600 to 700 g/m.sup.2. Usually the surface roughness required in the steel rolls is so severe that the sharp ridges on the roll wear very quickly and therefore exceed their usefulness in applying a roughened surface to the steel sheet.
Various techniques for surface roughening which do not necessitate rapid replacement of the steel rolls are described, for example, in Russian Patent SU 1,296,245. The work rolls provide a surface roughness of a depth of approximately 3.7 microns. Greater surface roughness which is achieved by blast treating the surface, for example with galvanized steel pipe, is described in Japanese published application 01/047,842 published Feb. 22, 1989. The blasted steel pipe has a surface roughness in the range of 20 to 80 microns before the galvanized material is applied.
Satin-finished surfaces, which provide a degree of roughened surface by virtue of a skin pass roll on the steel sheet is described in published Japanese application 59/104201 published Jun. 16, 1984. The satin finish provides a roughened surface on the steel sheet which has several peaks and regular corrugation to improve adhesion of the galvanized material to the steel sheet. Other types of skin pass rolling to achieve a surface roughness is described in published Japanese application 02/175,004 published Jul. 6, 1990. The skin pass rolling is achieved using rolls having pitted surfaces formed by laser dulling processing.
None of these processes however are capable of applying on a consistent basis, galvanized or other protective metallic coatings of a double-sided coating in excess of 600 g/m.sup.2. We have discovered that by forming a special pattern in the surface of the steel sheet a significant benefit in the amount of protective metallic coating can surprisingly be achieved.